Download Star Factories: Nuclear Fusion and the Creation of the Elements

Star Factories:
Nuclear Fusion and the
Creation of the Elements
Science In the River City workshop
3/22/11
Chris Taylor
Department of Physics and Astronomy
Sacramento State
Introductions!
Science Content Standards, Grades 9 - 12
Earth Sciences:
Earth's Place in the Universe
1.e ''Students know the Sun is a typical star
and is powered by nuclear reactions,
primarily the fusion of hydrogen to form
helium.''
2.c '' Students know the evidence
indicating that all elements with an
atomic number greater than that of
lithium have been formed by nuclear
fusion in stars.''
Three topics tonight:
1) how do we know all the heavier elements
are made in stars? (Big Bang theory)
2) How do stars make elements as heavy as or
less heavy than iron?
(Stellar nucleosynthesis)
3) How do stars make elements heavier than
iron? (Supernovae)
Big Bang Nucleosynthesis
The Big Bang theory
predicts that when the
universe first formed, the
only matter that existed
was hydrogen, helium,
and very tiny amounts of
lithium. If this is true,
then all other elements
must have been created
in stars.
Astronomers use
spectroscopy to examine
the light emitted by distant
stars to determine what
kinds of atoms are in them.
We've learned that most
stars contain nearly every
element in the periodic
table.
The spectrum of the Sun
In order to measure measure
what kinds of atoms were
around in the earliest days
of the Universe, we look
for stars that were made out
of fresh, primordial gas.
The closest we can get to this
is looking at dwarf galaxies,
which show extremely low
levels of elements heavier
than helium.
*
Stellar Nucleosynthesis
If heavy elements didn't get formed
during the Big Bang, then where do
they come from?
The Proton-Proton Chain
In fusion, atomic
nuclei are smashed
together at speeds high
enough to overcome the
electric repulsion of
their positive charges.
The nuclei then join
together to form a single
larger nucleus.
* The only place we know that fusion can
build heavy elements is inside the cores
of stars. The question now is:
What is it about the environment inside
stars that makes nuclear fusion possible?
High temperature; and
high density.
Demo
Stars like the Sun
will make helium
in their cores until
the hydrogen fuel
runs out. Some,
but not all, stars
will then switch
over to fusing
helium into
carbon. This is
the origin of all
carbon in the
Universe.
The higher the mass of the star, the heavier
elements it can create in its core. This is
because heavy element fusion requires higher
temperatures, which only the most massive
stars can attain.
Helium,
carbon and
oxygen.
Helium only
Helium and
carbon
The highest mass
stars can make all
elements up to and
including iron in
their cores.
But iron is the
heaviest element
they can make.
Fusion of iron does
not create energy,
and without an
energy supply, the
star will soon die.
*
Nucleosynthesis in Supernovae
If stars can only fuse elements upto and
including iron (Number 26 on the periodic
table) then where do all the gold, silver,
lead, uranium, etc... come from?
When a high mass
star's core runs out
of fusion fuel, the
core collapses,
causing the regions
near the core to fall
in and bounce off
the core.
This creates an
outward moving
shockwave of
atoms.
Demo
The R-Process
When the supernova explodes, large numbers
of neutrons are shot out of the interior of the
star at high velocities. Think of these like
pellets in a shot gun shell that has been fired.
These neutrons pass through the outer regions
of the star, colliding with the atoms already
there (mostly hydrogen). The collisions happen
very rapidly and quickly build up very large
atoms.
Fusion in stellar cores
Fusion in supernova explosions
Thanks!